Google has technology that tracks "identities" everywhere and establishes sophisticated database profiles. They know more about you and your interests, whether it's web activities or email topics, than you can fathom. Once you link gmail to a phone number and web surfing/shopping habits, it's there forever. Reading email on a phone or other devices establishes those relationships and contributes to your identify profile. Many are quite comfortable and accepting of this. I tend to be more old school and don't want to share private information.